This is the cheapest that i've found in UK. It's also a bronze rated seasonic oem so it's better quality than the corsair. Should be more than capable of running your rig, even with another graphics card added.

Yea. Without any modifications a 250w psu would be fine... If you plan on adding a graphics card later, I'd get at least 400w. If you plan on overclocking on top of that, I'd look for something closer to 500w.

Oh and psu brand is very important. I'd take a 500watt Seasonic over a 900watt generic any day.